Apparatus and process for improved hydrogen sulfide removal

ABSTRACT

Provided is a trap and method for removing hydrogen sulfide from a gas stream. The hydrogen sulfide trap includes a monolith substrate on which is disposed zinc oxide, and a second metal or oxide thereof. In some aspects, the hydrogen sulfide trap is advantageously incorporated into systems for producing hydrogen for PEM fuel cells.

[0001] This application is a continuation-in-part of U.S. Ser. No. 09/676,837 filed Sep. 29, 2000, the disclosure of which is herein incorporated by reference as if fully set forth herein.

[0002] The present invention relates to improved removal of hydrogen sulfide from gaseous mixtures. More particularly, the present invention relates to compositions, traps, and processes for decreasing levels of gaseous sulfur compounds, e.g., H₂S, in a gaseous stream by contacting the stream with compositions capable of absorbing such compounds.

[0003] Hydrogen sulfide occurs in various gas streams, for example, in sour natural gas streams and in tail gas streams from various industrial operations in which sulphur containing fuels and combustible materials are burned. Hydrogen sulfide is a highly toxic and odiferous substance, which is preferably substantially removed from gas streams before their ultimate discharge to the atmosphere. The toxicity of hydrogen sulfide is a problem of particular importance in the treatment of flue gases.

[0004] In addition, carbonaceous fuel and feedstock gases including natural gas and producer gas often contain sulfur components such as hydrogen sulfide. Even small amounts of hydrogen sulfide can poison metallic catalysts involved in conversion or utilization processes. For example, fuel processor systems that produce hydrogen for fuel cell applications, require that the gaseous sulfur compounds in a raw fuel stream be reduced to as low a level as practicable in order to avoid poisoning the catalysts such as steam reforming catalysts, water-gas shift catalysts, and the like. Furthermore, fuel cell electrodes can rapidly become inactivated due to gaseous sulfur compounds that contaminate the fuel stream, since the electrodes invariably contain precious metal components, such as platinum, that are extremely sensitive to the presence of sulfur compounds.

[0005] Conventional processes for hydrogen sulfide removal can be roughly divided into two categories: low temperature processes, e.g., below about 150° C. involving scrubbing the gas stream with liquid solutions (e.g., containing aqueous metal solutions that react with the H₂S in solution); and high temperature processes, e.g., about 300 to 500° C., involving contacting the gas stream with a bulk sorbent such as zinc oxide or ferric oxide.

[0006] To avoid contamination of metal containing catalysts, especially precious metal containing catalysts it would be desirable to have new devices and processes for hydrogen sulfide removal to lower H₂S concentrations in a hydrogen gas stream to levels below 100 ppb, and preferably below 20 ppb. That is, there is a need for devices and processes which will “polish” a partially desulfurized gas stream containing on the order of 1 ppm of H₂S and further decrease levels to below 100 ppb.

SUMMARY OF THE INVENTION

[0007] In one aspect, the invention relates to a hydrogen sulfide trap having a monolith substrate on which is disposed zinc oxide, and a second metal or oxide thereof dispersed on a support. The second metal or oxide thereof is selected from the group consisting of copper, nickel, iron, manganese, and combinations thereof. Preferably, the support includes zinc oxide.

[0008] A preferred hydrogen sulfide trap has a monolith substrate on which is disposed zinc oxide, and copper or an oxide thereof dispersed on a support. In one embodiment of the trap, the supported copper or oxide thereof forms a first layer adhering to the monolith substrate, and the zinc oxide forms a second layer overlying and adhering to the first layer. In another embodiment, the zinc oxide forms an upstream zone disposed on an upstream segment of the monolith substrate, and the supported copper or oxide thereof forms a downstream zone on a downstream segment of the monolith substrate.

[0009] In this preferred hydrogen sulfide trap coated with zinc oxide, and copper or an oxide thereof dispersed on a support, the copper or oxide thereof is preferably dispersed on a second portion of zinc oxide. In one embodiment of this hydrogen sulfide trap, the zinc oxide and the zinc oxide-supported copper or oxide thereof form a layer adhering to the monolith substrate. In another embodiment of this hydrogen sulfide trap, the zinc oxide-supported copper or oxide thereof is in the form of a first layer adhering to the monolith substrate, and the zinc oxide is in the form of a second layer overlying and adhering to the first layer. In one embodiment, the zinc oxide forms an upstream zone disposed on an upstream segment of the monolith substrate, and the zinc oxide supported copper or copper oxide thereof forms a downstream zone disposed on a downstream segment of the monolith substrate.

[0010] In another aspect, the invention relates to a process for removing hydrogen sulfide from a gas stream. The removal process includes contacting the gas stream with a hydrogen sulfide trap that has a monolith substrate on which is disposed zinc oxide, and a second metal or oxide thereof that is dispersed on a support. The second metal or oxide thereof is selected from the group consisting of copper, nickel, iron, manganese, and combinations thereof.

[0011] In a preferred embodiment of the process, the second metal or oxide thereof is copper. In another preferred embodiment, the second metal or oxide thereof is copper and the support is zinc oxide.

[0012] In another aspect, the invention relates to a system for producing hydrogen for a PEM fuel cell. The system has a hydrocarbon reformer reactor, a water-gas shift reactor, and a selective carbon monoxide oxidation reactor. The system also includes a monolith substrate on which is disposed zinc and a second metal or oxide thereof selected from the group consisting of copper, nickel, iron, manganese, and combinations thereof dispersed on a support. Preferably the second metal is copper. The monolith substrate is downstream and in train with the hydrocarbon reformer reactor, and upstream and in train with the water-gas shift reactor. A preferred system includes a monolith support having copper dispersed on a support of zinc oxide.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is a depiction of a channel of the monolith substrate coated with a single layer.

[0014]FIG. 2 is a depiction of a single channel of the coated monolith substrate having a double-layered configuration.

[0015]FIG. 3 is a depiction of a single channel of the coated monolith substrate having a zoned configuration.

[0016] The definitions of certain terms used herein are as follows:

[0017] “absorption” refers to processes wherein the chemical composition of the trapping material is changed. For example, the interaction between zinc oxide and hydrogen sulfide producing zinc sulfide is referred to as absorption.

[0018] “adsorption” refers to reversible association processes characterized as primarily surface phenomena, e.g., the interaction of copper or oxides thereof with hydrogen sulfide.

[0019] “inlet temperature” or “input gas temperature” shall mean the temperature of the hydrogen sulfide-containing stream being treated immediately prior to initial contact of the hydrogen stream, test gas, fluid sample or fluid stream with a trap composition.

[0020] “ppb” means 10⁻⁷ volume %

[0021] “percent by volume”, “volume percent” or “%v”, when used to refer to the amount of a particular gas component of a gas stream, unless otherwise indicated, means the mole percent of the gas component of the gas stream as expressed as a volume percent.

[0022] “support” refers to the material on which the second metal or oxide thereof is dispersed. Supports include high surface area supports such as inorganic oxide (e.g., alumina, zinc oxide).

[0023] “wt. %.”, unless otherwise indicated, means weight percent based on the weight of an analyte as a percentage of the total composition weight, including the support and any material impregnated therein. The percent by weight of a particular analyte (e.g., second metal oxide) of composite material (e.g. second metal oxide dispersed on an inorganic oxide support) is generally determined after impregnation with a suitable precursor of the analyte followed by calcination.

[0024] “VHSV” means volume hourly space velocity; that is, the flow of a reactant gas in liter per hour per liter of coated monolith substrate at standard temperature and pressure.

DETAILED DESCRIPTION OF THE INVENTION

[0025] The hydrogen sulfide trap, compositions and processes of the present invention are suitable for reducing concentrations of hydrogen sulfide in gaseous streams containing hydrogen sulfide gas. The compositions comprise a combination of zinc oxide and a second metal or oxide thereof dispersed on a support. In one aspect of the invention, Applicants have discovered that a combination of the zinc-containing component with a copper-containing component dispersed on a support can provide for removal of hydrogen sulfide from a gaseous stream to low levels, for example, levels below 100 ppb, and preferably below 20 ppb.

[0026] The hydrogen sulfide trap includes a monolith substrate. Unlike a fixed bed formed from particles of trap material, the monolith provides a body that can accommodate gas streams having high flow rates with minimal pressure drop across the monolith. The channel walls of the monolith substrates are coated with washcoat compositions that contain zinc oxide and a second metal or oxide thereof that has high affinity for hydrogen sulfide. The compositions therefore comprise trapping materials that can absorb hydrogen sulfide with enhanced efficiency. The zinc oxide is typically used in bulk form (i.e., not requiring dispersion on any support material) while the second metal or oxide thereof is typically dispersed on a support, preferably a high surface area support.

[0027] Second metals or oxides thereof with high H₂S affinity are preferably selected from copper, nickel, iron, manganese or combinations thereof. High surface area supports impregnated with these second metals or oxides thereof can be prepared, for example, forming an aqueous solution of a soluble salt of the second metal (e.g., copper nitrate, nickel acetate, and the like), impregnating the high surface area support with the solution, and calcining the resulting solids to form an oxide of the second metal. As will be appreciated by those of ordinary skill in the art, the second metal oxide can exist in multiple oxidation states after deposition and calcination on the substrate. By way of example, depending upon conditions Cu (metal), Cu₂O, and CuO may exist in the composition deposited on the monolith. These multiple oxidation states are within the scope of the invention. In preferred embodiments of the invention, the second metal with high H₂S affinity is copper or an oxide thereof.

[0028] The support, on which the second metal or oxide thereof is dispersed, includes high surface area supports. Useful high surface area supports are inorganic oxides, for example, silica and metal oxides, such as zinc oxide, ceria, titania, zirconia, alumina, oxides of iron, including mixed oxide forms such as silica-alumina, alumina-silicates which can be amorphous or crystalline, alumina-zirconia, alumina-chromia, alumina-ceria, and the like. In addition to inorganic oxides, activated carbon can also be used as a support.

[0029] Preferably the high surface area support is a quantity of the zinc oxide itself. In this preferred embodiment, the washcoat composition disposed on the monolith substrate therefore includes the second metal oxide dispersed on at least a portion of the zinc oxide.

[0030] In a preferred embodiment of the invention, the composition coating the monolith substrate includes copper or an oxide thereof dispersed on a support of zinc oxide. In certain embodiments, monolith substrates coated with copper or an oxide thereof dispersed on the zinc oxide display enhanced H₂S trapping absorption. While not being bound by theory, it is believed that the enhanced H₂S absorption is due to the intimate combination of copper oxide and zinc oxide. The copper oxide reacts rapidly with the H₂S in gas streams to form CuS, but only at the surface of the copper particles, as the copper has only a limited capacity for adsorbing H₂S. Zinc oxide, in contrast, has a higher capacity for absorbing H₂S due to the formation of bulk ZnS as well as the higher thermodynamic stability of the formed ZnS as compared to CuS in conditions where water is present. It is believed that the sulfur from an input gas stream is initially adsorbed onto copper oxide. Sulfur is subsequently transferred to zinc oxide to form zinc sulfide. The rapid adsorption of hydrogen sulfide by the copper component, in combination with the high capacity and thermodynamic stability of the zinc component thus provides for the enhanced effectiveness of the hydrogen sulfide removal.

[0031] In those embodiments having a washcoat containing copper or an oxide thereof dispersed on zinc oxide, the percent by weight of Zn is preferably at least 50% or greater (excluding binder components). More preferably the weight percent of Zn is at least 90% and most preferably about 99% (excluding binder components). Preferably, the zinc oxide-supported copper component in the form of micronized particles, more preferably as particles with an average diameter of about 10 μm or less.

[0032] As mentioned above, the washcoat compositions of the invention are disposed on monolith substrates to form layered monolith substrates. The monolith substrate is preferably of the type with one or more monolithic bodies having a plurality of finely divided gas flow passages (channels) extending therethrough. Preferably, the monolithic substrate (also referred to as a honeycomb substrate) is of the type having a plurality of fine, parallel gas flow passages extending across the longitudinal axis of the substrate from an inlet or an outlet face, so that the channels are open to fluid flow therethrough. The passages, which are essentially straight from the inlet and outlet of the substrates, are defined by walls on which the H₂S trapping material can be coated in washcoat compositions so that the gases flowing through the passages contact the trapping material.

[0033] Monolith substrates are commercially available in various sizes and configurations. The flow passages of the monolithic substrate are thin-walled channels which can be of any suitable cross-sectional shape and size such as trapezoidal, rectangular, square, sinusoidal, hexagonal, oval, circular. Such monolithic substrates may contain up to about 700 or more flow channels (“cells”) per square inch of cross section, although far fewer may be used. For example, the substrate can have from about 60 to 600, more usually from about 200 to 400, cells per square inch (“cpsi”).

[0034] Various types of materials of construction for monolith substrates are known. The monolith substrate can be made from a variety of materials, including metal or ceramic monoliths. In some embodiments, the monolith substrate can be made from a ceramic porous material composed of one or more metal oxides, e.g., alumina, alumina-silica, alumina-silica-titania, mullite, cordierite, zirconia, zirconia-ceria, zirconia-spinel, zirconia-mullite, silicon-carbide, and like. Some non-limiting examples of ceramic monoliths can include those made of: zirconium, barium titanate, porcelain, thorium oxide, magnesium oxide, steatite, boron or silicon carbonates, cordierite-alpha alumina, silicon nitride, spodumene, alumina-silica magnesia, zircon silicate, sillimanite, magnesium silicates, zircon, petalite, alpha alumina and aluminosilicates. One example of a commercially available material for use as the substrate for the present invention is cordierite, which is an alumina-magnesia-silica material.

[0035] In other embodiments, the monolith substrate can be made of a ceramic or metal foam. Monolith substrates in the form of foams are well known in the prior art, e.g., see U.S. Pat. No. 3,111,396 and SAE Technical Paper 971032, entitled “A New Catalyst Support Structure For Automotive Catalytic Converters” (February, 1997).

[0036] In some embodiments, a metallic monolith substrate can be used. The metallic monolith substrate can be a honeycomb made of a refractory metal such as a stainless steel or other suitable iron based corrosion resistant alloys (e.g., iron-chromium alloy). Metal monoliths can be produced, for example, from alloys of chromium, aluminum and cobalt, such as those marketed under the trademark KANTHAL, or those produced from alloys of iron, chromium, aluminum and yttrium, marketed under the trademark of FECRALLOY. The metal can also be carbon steel or simple cast iron. Monolith substrates are typically fabricated from such materials by placing a flat and a corrugated metal sheet one over the other and rolling the stacked sheets into a tubular configuration about an axis parallel to the configurations, to provide a cylindrical-shaped body having a plurality of fine, parallel gas flow passages, which can range, typically, from about 200 to about 1,200 per square inch of face area. Heat exchangers, which are typically formed from metallic materials, can also be used as the monolith structures.

[0037] The washcoat compositions comprising the H₂S trapping material can be disposed on the monolith substrates using a variety of coating architectures. The channel walls of the substrate can be coated, for example, with a washcoat composition of a mixture of the zinc oxide and supported second metal oxide material to form a single layer (11) on the channel walls (12), as shown in the depiction of a single channel (10) in FIG. 1. The thickness of the single layer can be controlled, in one method, by repeatedly coating the substrate with the same washcoat composition to obtain the desired thickness. In some embodiments a binder layer can be included to improve washcoat adhesion on the monolith, especially on metallic monoliths. A particularly preferred embodiment having this single layer architecture has a layer containing copper oxide supported on the zinc oxide.

[0038] In another configuration, the hydrogen sulfide trap is in the form of a layered composite disposed on the monolith substrate, as shown in the depiction of a single channel (20) in FIG. 2. In this configuration, the layered composite has a first layer (21) adhering to the channel walls (22), formed from a composition of the second metal oxide dispersed on a support. A second layer or top layer (23) containing zinc oxide overlies and adheres to the first layer. A gas stream flowing through the layered substrate would initially contact the second or top layer containing zinc oxide which is designed to remove the bulk of the hydrogen sulfide. The gas then passes to the first or bottom layer containing the supported second metal oxide to remove (or “polish”) the residual hydrogen sulfide.

[0039] A preferred embodiment having this layered architecture has a first layer formed from a composition containing copper oxide dispersed on a support, and a second layer formed from a zinc oxide composition. In a particularly preferred embodiment, the copper oxide is dispersed on a support of zinc oxide in the first layer.

[0040] An alternative coating architecture to the above-described layered composite is shown in the depiction of a single channel (30) in FIG. 3. Here, washcoat compositions containing separate trapping materials are disposed in discrete zones along the axial length of the substrate. In one preferred embodiment having this zoned architecture, for example, there is an upstream segment (as sensed in the direction of gas flow through the trap) of the substrate wall (32) coated with a composition of zinc oxide to form an upstream zone (31). A downstream segment of the substrate wall is coated with a composition containing the second metal oxide dispersed on a support to form a downstream zone (33). A gas stream flowing along the axial length of the substrate initially contacts the zinc oxide in the upstream zone of the substrate to remove the bulk of the hydrogen sulfide. The gas stream then passes through the zone coated with the supported second metal oxide composition (the downstream zone) to remove residual hydrogen sulfide.

[0041] A preferred embodiment having this zoned architecture has an upstream zone of the monolith substrate coated with zinc oxide and a downstream zone coated with copper oxide dispersed on a support. In a particularly preferred embodiment the copper oxide is dispersed on a support of zinc oxide in the downstream zone.

[0042] The washcoat slurries of the invention can be prepared using methods known in the art. In embodiments having discrete layers or zones of zinc oxide (i.e., unimpregnated by a second metal oxide), the layers or zones can be prepared from washcoat compositions or “slurries” of zinc oxide. For example, zinc oxide commercially available as an extrudate, is ball milled as a suspension using sufficient water to prepare a suspension of 30 wt. % solids. Thereafter, particle size distribution is measured. If 90% of the particles are <10 μm, the milling is complete; otherwise the milling is continued until such particle size has been achieved. Binders such as hydrated forms of alumina (e.g., pseudoboehmite), silica binders, clay binders, zirconia binders and the like are optionally included in the slurries to improve adherence of the washcoat to the substrate walls.

[0043] As mentioned above, washcoat compositions containing second metal oxides dispersed on a support can be prepared by first impregnating the support with soluble salts of the second metal, followed by a calcination step to convert the second metal component to its oxide. A preferred method of impregnating the support comprises mixing a solution of the second metal salt with finely-divided high surface area support that is sufficiently dry to absorb essentially all of the solution. For example, soluble salt forms of the second metal such as acetates, halides, nitrates, sulfates and the like can be utilized. The supported second metal material is then added to water and comminuted to form a slurry. Preferably the particle size of the all the solids in the slurry are <10 μm. Here again, binders can optionally be included in the slurries to improve adherence of the washcoat to the monolith substrate walls. The washcoat slurry can then be formed into a layer or zone on the monolith substrate. Once deposited on the substrate, the second metal component can be fixed onto the support (as the oxide) by calcining, preferably at temperatures above 300° C. Alternatively, the second metal component is fixed to its support by calcination before incorporation of the supported second metal component into a washcoat slurry and deposition on the substrate.

[0044] The washcoat slurries are applied to the substrate, for example, by methods well-known to those of ordinary skill. Thus, for example, a layer can be prepared by dipping the substrate in a reservoir containing a sufficient quantity of the slurry so that the substrate is fully immersed. The coated substrate can be dried and calcined. In embodiments, having a plurality of layers disposed on the substrate, a second or top layer can be applied on the first layer. Each layer can be calcined after each coating, or the monolith is calcined upon coating all of the layers.

[0045] In embodiments having substrates containing discrete zones coated with different trap material compositions, only an upstream longitudinal segment of the substrate would be dipped into a slurry of the zinc oxide slurry, and dried. The undipped longitudinal segment of the substrate would then be dipped into a slurry of the supported second metal component, and dried to form the downstream zone. The substrate is then calcined to fix the second metal component onto the support. In embodiments of the invention, wherein the monolith substrate is formed from metallic monolith substrates, the zones can be also be applied by depositing (e.g., sputtering) the washcoat slurries on either an upstream or downstream segment of the sheets before they are rolled up to form cylindrical monolith structures.

[0046] In another aspect, the present invention provides processes for removing hydrogen sulfide from a gaseous stream using the hydrogen sulfide traps of the invention. In the process, an input gas stream containing hydrogen sulfide is passed through a monolith substrate containing zinc oxide and a second metal oxide component dispersed on a support. In preferred embodiments of the process, the gas stream is treated with a substrate coated with zinc oxide and copper oxide dispersed on a support, preferably a support of zinc oxide.

[0047] Advantageously, the space velocities of the input gas streams can be quite high due to significantly lower pressure drops across monolith substrates, as compared to fixed beds of particulate trap material. For example, the space velocity of the gaseous stream can range from about 300 hr⁻¹ to over 100,000 hr⁻¹ across the substrate, depending on the particular application desired. Preferably the space velocity of the gaseous stream is from about 2,000 hr⁻¹ to about 20,000 hr⁻¹.

[0048] The processes of the invention can effectively treat gas streams of different temperatures, so that a gas streams arising from different sources/applications can be accommodated. For example, the processes and hydrogen sulfide traps can effectively treat gas streams having temperatures below about 550° C., preferably between about 250° C. and 500° C. In the absence of steam, lower temperatures may be preferred.

[0049] Preferably, the input gas stream comprises less than 10 ppm of hydrogen sulfide, more preferably less than 1 ppm and most preferably less than 100 ppb, so that the processes effectively provide output gas streams having <20 ppb of H₂S.

[0050] The processes of the present invention operate most efficiently when the gaseous sulfur components in the gas stream prior to contact with the trap is primarily H₂S. In other words, the input gas stream preferably has an environment wherein the stream is substantially free of oxidized forms of sulfur such as SO₂ and SO₃.

[0051] Other gaseous component that can be present in the input gas stream include any component that is substantially inert to the zinc oxide and second metal oxides dispersed on the support. For example, hydrogen, steam, carbon monoxide, carbon dioxide, and hydrocarbons are all typically present in fuel processor systems that generate hydrogen for fuel cells.

[0052] Advantageously, the devices of the invention can be used to lower or “polish” a gaseous stream containing about 100 ppb of hydrogen sulfide to lower hydrogen sulfide levels. When input gas streams containing 100 ppb of hydrogen sulfide at space velocities of 10,000 hr⁻¹ are treated with monoliths having the coating compositions of the invention, the hydrogen sulfide concentration of hydrogen sulfide in the output stream is preferably less than 50 ppb, more preferably less than 20 ppb. In embodiments where an input gas stream is treated with a monolith having a coating of copper oxide dispersed on zinc oxide, levels of hydrogen sulfide as low as 5 ppb or lower can be expected.

[0053] Although the hydrogen sulfide removal devices can be used in any application where low H₂S levels are needed, a particularly useful application is in systems such as fuel processors that provide hydrogen to fuel cells. These systems typically comprise a series of reactors that convert hydrocarbon fuels (e.g., natural gas, gasoline, fuel oil, liquid petroleum gas, and the like) into hydrogen fuel. The conversions that take place in the reactors typically include reforming reactions and water gas shift reactions to produce hydrogen. Other reactors and trapping apparatus can also be included in the system that reduce unwanted components in the hydrogen feed streams, such as carbon monoxide and other sulfur components, that are ultimately supplied to the fuel cell.

[0054] The reforming reactor is typically the first site at which carbonaceous fuels (specifically hydrocarbons) are converted, at least in part, to hydrogen as well as other products including carbon monoxide. The metals of the catalysts of the reforming catalysts that include precious metals are generally are generally tolerant of low levels of hydrogen sulfide. Catalysts (e.g. platinum group metals, and base metals such as copper and nickel) in the reactors downstream of the reforming reactor in the system, however, are poisoned by even small amounts of hydrogen sulfide. In addition, as mentioned above, platinum electrodes used in PEM fuel cells are also damaged upon exposure to small amounts of hydrogen sulfide. Incorporating a hydrogen sulfide trap of the invention into the fuel processor system downstream of the reforming reactor and upstream of the water-gas shift reactor can protect the catalysts of the downstream reactors (e.g., the water-gas shift reactor) as well as the electrodes of PEM fuel cells from poisoning by hydrogen sulfide.

[0055] The following examples further illustrate the present invention, but of course, should not be construed as in any way limiting its scope. Unless otherwise indicated, all amounts and percentages are on a weight basis.

EXAMPLE 1 Preparation of Coated Monolith Substrates Preparation of a Monolith Substrate Coated with a Composition of CuO Dispersed on a ZnO Support (CuO/ZnO)

[0056] A slurry of Cu(NO₃)₂ on zinc oxide was prepared by adding a 5 M solution of Cu(NO₃)₂ (224.1 g of the solution) to transparent ZnO (201.1 g, from Bayer AG) to incipient wetness. The resulting material was dried overnight at 120° C., to a dry weight of 312.64 g. Pseudoboehmite (50 g) was added. The resulting powder was transferred to a jar mill and deionized water (300 mL) was added. The slurry was milled until the particle size distribution was less than 10 microns.

[0057] The slurry was coated onto a 400 cpsi monolith substrate having a diameter of ¾ in and a height of 3 in. The coated monolith substrate was calcined for 2 h at 120° C. and then for 2 h at 400° C. The final loading of the CuO/ZnO was determined to be 1.89 g/in³ (based on weight of the uncoated substrate, final weight of the calcined substrate, and the volume of the substrate). The monolith substrate was cut into two pieces, each piece having a height of 1.5 in, before its evaluation in Example 2.

Preparation of a Layered Composite on a Monolith Substrate with a ZnO Layer and a Layer of CuO Dispersed on a ZnO Support (ZnO—CuO/ZnO)

[0058] A first layer (bottom layer) was formed from a washcoat slurry of CuO dispersed on a ZnO support having a loading of 1.68 g/in³ after calcination of the monolith substrate (0.75 in×3 in, 400 cpsi). In this first layer, powdered ZnO (Azo-66) was soaked with an aqueous solution of (NH₄)₂CO₃ before impregnation with Cu(NO₃)₂.

[0059] A second layer was formed, overlying the first layer, from a washcoat slurry containing transparent ZnO (from Bayer AG) and 20% pseudoboehmite. The second layer had a zinc oxide loading of 0.41 g/in³. The resulting layered monolith substrate was cut into two pieces, each having a height of 1.5 in, before its evaluation in Example 2.

Preparation of a Comparative Example (ZnO Coated Monolith Substrate)

[0060] A monolayer was formed on the monolith substrate (0.75 in×3 in, 400 cpsi) from a washcoat slurry containing transparent ZnO (from Bayer AG). After calcination, the loading of the ZnO was determined to be 1.55 g/in³. Here again, the resulting monolith substrate was cut into two pieces, each having a height of 1.5 in, before its evaluation in Example 2.

EXAMPLE 2 H₂S Removal from a Gas Stream

[0061] The coated monolith substrates from Example 1 were mounted in the center of a 24-in long quartz reactor tube which contained a quartz frit at the distal region of the tube. On the quartz frit was placed 4 g of a granulated 12% CuO on activated alumina (300-700 μm particle size) to capture the sulfur breaking through the monolith. (The 12% CuO on alumina, was prepared by impregnating alumina particles with the appropriate amount of Cu(NO₃)₂ solution followed by drying at 120° C. in air and calcination at 500° C. in air.)

[0062] A gas stream containing 40% v/v hydrogen in nitrogen admixed with 25% v/v steam and 4 ppm H₂S was passed over the CuO/ZnO monolith and then over the CuO/alumina at a temperature of 350° C. and a flow rate of 1.5 L/min (space velocity of 11,000 hr⁻¹ VHSV for this particular monolith). After 24 hours of operation, the gas flow was stopped, and the CuO/alumina was removed and analyzed for its sulfur content. It was assumed that the CuO/alumina absorbs all sulfur passing through the monolith. The exit concentration of H₂S after the monolith was calculated from the sulfur content of the CuO/alumina material after accounting for the inherent H₂S on a blank of CuO on alumina (identical 4 g quantity). The analytical technique used to determine sulfur content in the CuO/alumina analyte was oxidation of the H₂S in the analyte, followed by quantitative infrared spectroscopy. Results of the experiment are shown in Table 1 below. TABLE 1 total ppm Sulfur total sulfur sulfur ppb H₂S Sample on on sample flow % sulfur after Description CuO/Al₂O₃ (mg) (mg) adsorbed monolith ZnO (1.55 228 114 6193 98.16% 73.63 g/in³⁾ CuO/ZnO 95.5 47.75 6193 99.23% 30.84 (1.89 g/in³⁾ ZnO (0.41 52.5 26.25 6193 99.58% 16.95 g/in³)- CuO/ZnO (1.55 g/in³⁾

[0063] As can be seen in Table 1, the combination of a monolith substrate coated with CuO on a ZnO support (row 2) significantly decreased the amount H₂S detected in the analyte (Cu/alumina material) over a simple ZnO coated monolith substrate (row 1). A further decrease in the breakthrough quantity of H₂S detected in the analyte was observed in the bilayer monolith substrate (row 3) having a first layer of CuO dispersed on a ZnO support and a second layer of ZnO. Even at high space velocities, the enhanced removal of H₂S was observed.

[0064] While this invention has been described with an emphasis upon preferred embodiments, it will be obvious to those of ordinary skill in the art that variations in the preferred devices and methods may be used and that it is intended that the invention may be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications encompassed within the spirit and scope of the invention as defined by the claims that follow. 

What is claimed:
 1. A hydrogen sulfide trap comprising: a monolith substrate on which is disposed, zinc oxide; and a second metal or oxide thereof selected from the group consisting of copper, nickel, iron, manganese, and combinations thereof dispersed on a support.
 2. The hydrogen sulfide trap of claim 1, wherein the support is selected from the group consisting of zinc oxide, ceria, titania, zirconia, alumina, oxides of iron and combinations thereof.
 3. The hydrogen sulfide trap of claim 1, wherein the support comprises zinc oxide.
 4. A hydrogen sulfide trap comprising a monolith substrate on which is disposed zinc oxide, and copper or an oxide thereof dispersed on a support.
 5. The hydrogen sulfide trap of claim 4, wherein the supported copper or oxide thereof forms a first layer adhering to the monolith substrate, and the zinc oxide forms a second layer overlying and adhering to the first layer.
 6. The hydrogen sulfide trap of claim 4, wherein the zinc oxide forms an upstream zone disposed on an upstream segment of the monolith substrate, and the supported copper or oxide thereof forms a downstream zone on a downstream segment of the monolith substrate.
 7. The hydrogen sulfide trap of claim 4, wherein the support comprises a second portion of zinc oxide.
 8. The hydrogen sulfide trap of claim 7, wherein the zinc oxide and the zinc oxide-supported copper or oxide thereof form a layer adhering to the monolith substrate.
 9. The hydrogen sulfide trap of claim 7, wherein the zinc oxide-supported copper or oxide thereof is in the form of a first layer adhering to the monolith substrate, and the zinc oxide is in the form of a second layer overlying and adhering to the first layer.
 10. The hydrogen sulfide trap of claim 7, wherein the zinc oxide forms an upstream zone disposed on an upstream segment of the monolith substrate, and the zinc oxide supported copper or copper oxide thereof forms a downstream zone disposed on a downstream segment of the monolith substrate.
 11. The hydrogen sulfide trap of claim 4, further comprising a binder.
 12. A process for removing hydrogen sulfide in a gas stream comprising hydrogen sulfide, the process comprising: contacting the gas stream with a hydrogen sulfide trap comprising a monolith substrate on which is disposed; zinc oxide, and a second metal or oxide thereof selected from the group consisting of copper, nickel, iron, manganese, and combinations thereof dispersed on a support.
 13. The process of claim 12, wherein the second metal or oxide thereof is copper.
 14. The process of claim 12, wherein the second metal or oxide thereof is copper and the support is zinc oxide.
 15. The process of claim 14, wherein the zinc oxide and the zinc oxide supported copper or oxide thereof is in the form of a layer.
 16. The process of claim 12, wherein the gas stream has a space velocity of at least 300 hr^(−1.)
 17. The process of claim 12, wherein the gas stream has a temperature below about 500° C.
 18. The process of claim 12, wherein the gas stream has a sulfur concentration of no more than 10 ppm.
 19. The process of claim 18, wherein the gas stream has a sulfur concentration of no more than 100 ppb.
 20. In a system for producing hydrogen for a PEM fuel cell, the system having a hydrocarbon reformer reactor, a water-gas shift reactor, and a selective carbon monoxide oxidation reactor, the improvement comprising: a monolith substrate on which is disposed zinc and a second metal oxide selected from the group consisting of copper, nickel, iron, manganese, and combinations thereof dispersed on a support, wherein the monolith substrate is downstream and in train with the hydrocarbon reformer reactor, and upstream and in train with the water-gas shift reactor.
 21. The system of claim 20, wherein the second metal or oxide thereof is copper.
 22. The system of claim 21, wherein the support comprises a second portion of zinc oxide. 